Joking with the audience at her debut ­after-dinner speech, Pippa spoke for the first time about the frock and admitted: “As I have found out, recognition has its upside, its downside and – you may say – its backside.”

The 30-year-old revealed she was so nervous before Kate’s wedding to Prince William that she did not sleep for two nights.

Around 400 people were at the Women in Advertising and Communications dinner at Central London’s swanky Savoy Hotel, with tickets costing £132 each.

One guest tweeted:

Wearing a floor-length dark green number, she told the crowd that her life had changed forever after the wedding in April 2011.

She admitted accusations that she was cashing in on her royal connections by writing a book and accepting offers to write columns for magazines and newspapers, had hurt her.

Pippa also said the negative response to her party planning book Celebrate – which only sold 18,000 copies – had been “difficult”.

A guest said: “It was the first time she’s made a speech of this kind but you wouldn’t have known it, she was so relaxed. It was unbelievable how open she was – I don’t think anyone was expecting that.

“She talked about the royal wedding and her career since. She came across as really grateful for the opportunities but also talked about the pitfalls of her position.”

Pippa – currently taking advice from PR firm Seven Dials, run by the former Independent editor Simon Kelner – followed in the footsteps of Labour leader Ed Miliband, comedian Ruby Wax and movie director Danny Boyle as a speaker for WACL.